In the blowing of a foil, a synthetic resin (plastic) is extruded from an angular nozzle in the form of a tube at a temperature and under conditions in which the extruded material retains its plastic state so that, upon blowing, the tube is expanded into a bubble or balloon and hence the wall thickness is reduced to a thickness of a foil. The blowing head is usually oriented vertically with the tube extruded upwardly and the balloon closed at the upper end thereof, after the foil has cooled, to close off the balloon in the nip between a pair of rollers.
The result is either a double-layer foil which can be wound into a roll or which can be split into individual layers directed in opposite directions to be wound in single-layer turns in respective rolls.
From the prior art systems serving as the systems upon which this invention is an improvement, the annular gap nozzle is formed with an outer and/or an inner nozzle lip of very limited axial length and very small thickness so that the lip can be radially deformable and can be provided (see German patent document 34 17 912 and U.S. Pat. No. 4,592,710) over its periphery with positioning elements which are equidistantly distributed and radially effective to displace the lip locally. The positioning elements are so-called field translators, i.e. electrically controllable positioning elements which are in the form of piezoelectric or magnetostrictive elements.
The result is the ability to control the orifice or gap width from which the thermoplastic tube is extruded to effect thickness correction in the foil which is ultimately formed.
This earlier system has proved to be effective in many cases, but the use of field translators has limited the versatility since they cannot generate very large forces and hence require very thin lips.
It is also known, in this context (see German patent document 40 13 610 and copending U.S. patent Ser. No. 07/598,392 filed 16 Oct. 1990 which is now U.S. Pat. No. 5,110,518) to provide an elastically-deformable nozzle lip which can be placed under prestress in the radial direction by appropriate positioning elements capable of being varied to effect thickness error correction. The use of an initial prestress which can thus be varied has been found to be especially effective for thickness control of the annular gap of the nozzle and especially local thickness adjustment.